Foraging strategies of a pursuit-diving seabird in a dynamic marine environment

[English]
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Abstract

Flexible foraging tactics are critical to survival for predators foraging in dynamic environments. This is especially true for breeding seabirds as they have to fuel their own energy needs and those of their offspring from a fixed breeding site. To succeed parents must employ foraging strategies that maximize overlap with prey through a range of environmental conditions.
Off Newfoundland, breeding common murres Uria aalge specialize on capelin Mallotus villosus. The spatial and temporal distribution of capelin is, therefore, expected to influence the foraging decisions of murres. Though capelin are predictable in many ways, several physical challenges (travel time, light limitations, etc.) limit the murres’ foraging activities. This research focuses on how common murres breeding in Newfoundland deal with their foraging ocean-scape. Integrated analyses of tracking data, colony-based observations, vessel surveys of prey, and physical environmental measurements were used to assess this question.
To deal with environmental change, murres exhibited remarkable behavioural flexibility. Though diel vertical migrations of capelin challenged their diving and visual abilities, murres successfully captured capelin throughout diurnal, crepuscular (twilight), and nocturnal periods. They used moonlight to capture capelin at night and also appeared to rely on non-visual cues to capture capelin under a starlit sky – when virtually no light is available in the water column. Over horizontal scales, murres appear to rely on memory to relocate successful foraging patches and area-restricted search to refine their foraging efforts. Because capelin tend to occur in predictable patches that are ephemeral at fine scales, this is likely the most efficient strategy. In combination with flexible time budgets, these strategies likely help murres buffer chick-feed rates across a range of prey densities. However, during mismatch between chick-rearing and peak capelin availability, parental murres were unable to fully compensate for limited access to prey and breeding success was consequently reduced. Though murres exhibit remarkable behavioural plasticity, there are limits to their plasticity. Revealing the limits improves our mechanistic understanding of the links between environmental variability and population dynamics.